In recent years, a great variety of electrical appliances have become available for operation by the low voltage power of direct-current electrical systems found in vehicles. Such appliances are generally interconnected to the electrical system by use of an adapter plug or connector, which is inserted into a cigarette lighter socket or receptacle. Electrical appliances and accessories that can be operated from a vehicle electrical system include battery chargers, portable televisions, cellular phones and the like.
There are generally two standard diameters of cigarette lighter sockets. The American and Japanese standard is about 20.9 millimeters, while the European standard is about 22.3 millimeters. Known adapter plugs are designed for use with only one standard receptacle size. Each prior art design includes by necessity dimensions tailored to the particular socket size intended for use. It is therefore desirable to design an adapter plug that is compatible with a plurality of standard sizes and which will not loosen or eventually lose electrical contact when subject to vibration or shock when positioned within any of the standard socket diameters.
In general, existing adapter plug designs each have only one or two spring contacts for mating with the socket receptacle. Some plug designs have placed contacts in varying positions around the plug surface to provide an offset force and thereby more soundly hold the plug in place.
If there are two or more contacts, they are typically angularly spaced around the periphery of an adapter plug. For example, U.S. Pat. No. 4,988,315 to Wharton, incorporated herein in its entirety by reference, discloses an adapter plug having two spring contacts radially positioned opposite each other at an angle of less than 180 degrees.
Another prior art design includes a cylindrical plug sleeve which slidably fits around the tubular adapter plug body perimeter to broaden the diameter of the plug body. However, the plug sleeve presents the user with the inconvenience of determining whether use of the sleeve is necessary, locating the sleeve and inserting the sleeve over the plug body in the proper position. Also, the sleeve adds a significant cost to the manufacture and shipment of the adapter plug.
Thus, none of these designs self-adjusts to a variation in the diameter of the socket. In a larger socket, a plug designed for a smaller diameter will be free to pivot around the contact points which is likely to cause the plug to loosen and eventually break electrical contact with the electrical system when subjected to vibration or jarring. Conversely, a plug designed for a larger diameter will not fit into a small standard sleeve size.